With the development of data centers, an input of the point-of-load (POL) may be converted from 12V to 48V, or even up to 400V, while generally an output voltage of the POL is relatively lower. Accordingly, a high-frequency converter circuit having a high input/output voltage ratio is required, and a transformer is generally included in such a circuit. However, it is also difficult to realize high efficiency under a high-frequency high current at the output side. In the related art, for example, an LLC (resonant converter) topology is widely used due to its low switching loss, high efficiency, and easily extensible output power. In an application with a low-voltage-high-current output, as illustrated in FIG. 1, generally a plurality of transformers are connected in series at the high voltage side and in parallel at the low voltage side. The circuit illustrated in the dashed block shown in FIG. 1 may be defined as a transformer unit. As illustrated in FIG. 2, the transformer unit may include a primary winding at the high voltage side and a secondary winding at the low voltage side. As illustrated in FIG. 2, the low voltage side further includes two switching devices. A transformer is an essential electromagnetic element in a 48V/400V level application of the board level of the data center, and optimizing the transformer is a key factor for improving efficiency.
The transformer unit illustrated in FIG. 2 is a transformer winding structure with taps, which includes a primary winding W3, secondary windings W1 and W2 and switching devices Q1 and Q2. The secondary windings W1 and W2 are connected via at least one via hole C, and the switching devices Q1 and Q2 are connected via at least one via hole D. Alternate current (AC) flows in a loop formed by the series connected secondary windings W1 and W2 and switching devices Q1 and Q2. In a high frequency application, lowering an impedance of the AC loop at the low voltage side is a key factor for optimizing winding loss and inductance leakage of the transformer.
Currently, in a winding structure of a low-voltage-high-current transformer, most windings are arranged in an S/P/S structure or an S/P/P/S structure. With respect to the transformer unit of FIG. 2, it is implemented in a W1/W3/W2 structure, in which the secondary windings W1 and W2 are positioned in different layers of a print circuit board (PCB) and are nonadjacent, and an interlayer between the secondary windings W1 and W2 is a primary winding W3. The secondary windings W1 and W2 are connected via at least one via hole C. In order to reduce inductance leakage and winding loss of the transformer, the AC loop at the low voltage side is preferably disposed within a projection of the primary winding. As illustrated in FIG. 3, typically, switching devices Q1 and Q2 are disposed as close as possible to the projection of the primary winding W3, and the via hole C between the secondary windings W1 and W2 are disposed as close as possible to the projection of the primary winding W3. As illustrated in FIG. 4, for example, in some products, one of the via holes between the secondary windings W1 and W2 may be provided within the projection of the primary winding W3, so as to shorten the high frequency current loop and reduce the impedance of the loop, thereby reducing winding loss. It can be analyzed that in such a transformer winding structure, the winding loss may be reduced about 20% by introducing one such via hole. However, in such a transformer of the S/P/S structure, there cannot be too many such via holes since via holes between the secondary windings W1 and W2 will pass through the primary winding. Otherwise, the primary winding may be cut off, such that primary winding loss may be increased. Accordingly, as the number of via holes is limited, further optimization of the winding loss is also limited.
It should be noted that, information disclosed in the above background portion is provided only for better understanding of the background of the present disclosure, and thus it may contain information that does not form the prior art known by those ordinary skilled in the art.